In general, thermal efficiency of a heat engine increases as a compression ratio becomes higher. Here, the compression ratio refers to a ratio of a volume of air which is compressed in a cylinder by a piston. That is, if a volume of a combustion chamber is changed, the compression ratio may be changed.
By changing the compression ratio in accordance with an operational state of an engine, fuel efficiency may be improved in a low load condition of the engine by raising the compression ratio of an fuel-air mixture, and the occurrence of knocking may be prevented and engine output may be improved in a high load condition of the engine by lowering the compression ratio of the fuel-air mixture.
In the related art, a variable compression ratio (VCR) apparatus, which changes the compression ratio by changing a top dead center of the piston, has been used.
However, the variable compression ratio apparatus of the related art requires a power source such as a plurality of links, and a motor to change the top dead center of the piston. Therefore, a weight of the engine is increased due to weight of the links and the motor, and a complicated design is required to prevent interference of the plurality of links. In addition, in a case in which it is not easy to change a volume of compressed air, it may be difficult to change the compression ratio.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.